FIG. 1 is a cross-sectional view showing a schematic structure of a prior art gain-coupled semiconductor DFB laser device which is shown in an article "Fabrication and Characteristics of a Gain-Coupled Distributed-Feedback Laser Diode" by Yi Luo, Yoshiaki Nakano, and Kunio Tada, in SSDM (SOLID STATE DEVICES AND MATERIALS) Aug. 24-26, 1988, pages 327-330.
In FIG. 1, on an N+-type GaAs substrate 1, an Al.sub.0.35 Ga.sub.0.65 As lower cladding layer 2, an undoped GaAs active layer 3, and a P-type Al.sub.0.30 Ga.sub.0.70 As carrier confinement layer 4 are stacked in the named order. On the carrier confinement layer 4, a P-type GaAs waveguide layer 5 is disposed. The P-type GaAs waveguide layer 5 includes in its surface a second order diffraction grating corrugation 51 with a period t equal to, for example, 255 nm, formed by reactive ion etching (RIE). On the waveguide layer 5, a P-type Al.sub.0.35 Ga.sub.0.65 As upper cladding layer 6 and a P+-type GaAs contact layer 7 are stacked in the named order.
When a bias voltage of suitable magnitude is applied between the P+-type GaAs contact layer 7 and the N+-type GaAs substrate 1, light is generated in the undoped GaAs active layer 3. This light is fed back by a perturbation in gain or loss coefficient due to the presence of the P-type GaAs waveguide layer 5 with the second-order diffraction grating corrugation 51 formed therein, and, therefore, only a single wavelength is selected and emitted.
As stated above, light generated in the undoped GaAs active layer 3 is fed back by a perturbation in gain or loss coefficient provided by the second-order diffraction grating corrugation 51. This is the reason a DFB laser device of this type is called gain-coupled DFB laser device.
In addition to the above-stated gain-coupled type device, DFB laser devices include a refractive-index-coupled DFB laser device like the one disclosed in Japanese Unexamined Patent Publication No. SHO 62-166582. In this refractive-index-coupled DFB laser device, light is fed back by a perturbation in the index of refraction which is provided by disposing, near an active layer, a corrugation formed by a material which is transparent to an oscillation wavelength.
The refractive-index-coupled DFB laser device has been long developed and has become practically usable. However, the refractive-index-coupled DFB laser device has a problem that it tends to produce a pair of longitudinal oscillation modes having the same threshold gain. Therefore, in order to obtain laser light of single longitudinal mode in the refractive-index-coupled DFB laser device, it is necessary to provide coatings that are extremely asymmetric in reflectivity with respect to each other, such as a front end coating having a reflectivity of 1% and a rear end coating having a reflectivity of 95%. Furthermore, even with such asymmetric coatings, the probability that the refractive-index-coupled DFB laser device will oscillate in a desired single longitudinal mode is about 50-70%.
In contrast, in general, gain-coupled DFB laser devices, including the one shown in FIG. 1, have the advantage that DFB laser devices oscillating in a single longitudinal mode can be fabricated with a good yield, because the gain-coupled DFB laser device has essentially only one longitudinal mode of oscillation having a minimum threshold gain.
As described above, however, in the conventional gain-coupled semiconductor DFB laser device shown in FIG. 1, because light is fed back by a perturbation in gain or loss coefficient provided by the P-type GaAs waveguide layer 5 with the second-order diffraction grating corrugation 51, and the corrugation 51 is continuous, the internal loss within the laser device is large, which results in an oscillation threshold current that is large and a light-emitting efficiency that is low.
Some prior art DFB laser devices with a single-longitudinal-mode oscillation achieved by feeding back light by a perturbation in loss coefficient, include a semi-insulating epitaxial layer as a current blocking layer. This laser device. however, has the disadvantage that its internal loss is large because of a number of deep energy levels present within a semi-insulating epitaxial layer.